While the prime minister's seemingly candid commentary may prove the more awkward misstep in the realm of diplomacy (leaders of both Nigeria and Afghanistan will attend an anti-corruption summit in London this week), it was the Queen's highly unusual, unguarded remarks that came as a bigger surprise.

Given her role in Britain's constitutional monarchy, the Queen simply doesn't offer her opinion on matters of governance or diplomacy very often -- and in her 63 years on the thrown, she's become renowned for maintaining that public stoicism.

As D'Orsi explained to the Queen how it had been a trying time in her career, with the Chinese delegation threatening to cut the visit off in a private meeting with the police commander and Britain's ambassador to China, the 90-year-old head of state agreed; "They were very rude to the ambassador."

The videotaped encounter came just several hours after a Buckingham Palace reception with the Queen during which a television microphone caught Cameron saying "we have got some leaders of some fantastically corrupt countries coming to Britain."

He referred to "Nigeria and Afghanistan -- possibly two of the most corrupt countries in the world."

Cameron will bring together politicians, officials, businesspeople and civil-society groups from around the world Thursday as part of efforts to clamp down on bribery, money-laundering and other forms of wrongdoing.

The Speaker of the House of Commons John Bercow (L) speaks as Queen Elizabeth, Archbishop of Canterbury Justin Welby, Prime Minister David Cameron and Chris Grayling, leader of the House of Commons listen, at a reception in Buckingham Palace to mark the The Queen's 90th Birthday, London, Britain May 10, 2016.
REUTERS

Both are due to attend Thursday's meeting, which hopes to produce a global declaration against corruption and break what Cameron has called the "taboo about tackling this issue head-on."

Buhari spokesman Garba Shehu reacted on social media, using an official account to say Cameron was embarrassing Nigerians despite Buhari's "good work" on fighting corruption.

"The prime minister must be looking at an old snapshot of Nigeria. Things are changing with corruption and everything else," he wrote.

Cameron's 10 Downing St. office said the prime minister knew there were "multiple cameras in the room" when he spoke.

A spokesman said that Buhari and Ghani "have been invited to the summit because they are driving the fight against corruption in their countries. The U.K. stands shoulder to shoulder with them as they do so."